. The chordates. Chordata. Abd. \. Fig. 493. Diagrams illustrating the descent of the testis as seen in parasagittal section, (d. def.) Ductus deferens; (Proc. Vag.) processus vaginalis (the diver- ticulum of the peritoneum pushed into the scrotal sac). (Courtesy, Patten: "Em- hryology of the Pig," Philadelphia, The Blakiston Company.) known as cryptorchism—and so must remain permanently in the abdominal cavity. In some mammals (, tapir, rhinoceros, civet, otter) each testis lies in a bursa which extends more or less deeply into the inguinal or perineal (Fig. 494E) region, but th
. The chordates. Chordata. Abd. \. Fig. 493. Diagrams illustrating the descent of the testis as seen in parasagittal section, (d. def.) Ductus deferens; (Proc. Vag.) processus vaginalis (the diver- ticulum of the peritoneum pushed into the scrotal sac). (Courtesy, Patten: "Em- hryology of the Pig," Philadelphia, The Blakiston Company.) known as cryptorchism—and so must remain permanently in the abdominal cavity. In some mammals (, tapir, rhinoceros, civet, otter) each testis lies in a bursa which extends more or less deeply into the inguinal or perineal (Fig. 494E) region, but the two sacs do not meet in a common scrotum and they cause little or no external protrusion of the body- wall. In marsupials the scrotum, lacking in some of them, is anterior to the penis. In placental mammals the positions are reversed—the scrotum is posterior. "Inguinal canal" is a name which may be applied logically and conveniently to the communicating passage between the abdominal cavity and the bursa inguinalis, and is often so used, but confusion results from the fact that the same name is applied, especially in human anatomy, to the passage occupied by the spermatic cord in its course through the abdominal wall after the former passage has become closed. The former (Fig. 493C) connects two regions of the coelom and is internal to the peritoneum. The vas deferens and other structures which constitute the spermatic cord lie in the dorsal wall of the bursa inguinalis and primarily external to the tunica vaginalis (peritoneum). In strict sense, the two passages are not the same, although intimately related. The mechanics of the "descensus testiculorum" is a problem not yet fully solved. Apparently several factors are involved. The initial factor in the backward shifting of the gonads, both ovary and testis, seems to be a polarized growth. Persistent growth at the hind end of the gonad is accompanied by atrophy, at a compensating rate, of the anterior
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